When a projectile is fired from a gun barrel, the gases generated by the ignition of the propellant impart a large force which accelerates the projectile down the barrel. As the projectile travels down the barrel, the expansion effect from the burning propellant will increase until a maximum pressure is reached and afterwards, the pressure drops rapidly. To optimize the projectile's performance, its acceleration down the gun barrel should be maximized in the gun barrel for as long as possible without exceeding the mechanical stress limits of the gun. This may be achieved by designing a propellant with a progressive gas generation rate or “progressivity” which optimizes the work imparted on the projectile by the propellant gases as it travels down the gun tube in order to maximize its acceleration characteristics. An ideal progressivity is sought that can accelerate a projectile down the gun tube while reducing the risk of gun tube failure from over-pressurization.
One way to achieve the ideal progressivity is by using two chemically different propellant formulations possessing two markedly different gas generation rates and energy densities. In this design, only the slower burning propellant layer is exposed to the ignition source initially, and the faster burning layer is exposed to the ignition source only after all of the slower layer has been consumed. If the exposure of faster burning layer is properly timed and the gas generation rate differential is large enough, then the gun pressure will reach its peak, subside, rise to a peak again and then subside as the projectile travels down the gun tube. This phenomenon will be depicted as a “double hump” in the pressure-time (P-t) curve, which is characterized by two local peaks and wider shaped curves near the peak pressure when compared to the traditional propellant. The area under the P-t curve is correlated to the pressure-volume (P-V) work of the expanding gas done on the projectile, hence the propulsion system with a wider P-t curve that remains below the permissible pressure limit of the gun tube will result in a higher kinetic energy projectile.
There are multiple ways to manufacture multi-layered propellant compositions having different gas generation rates. One way is to fabricate thin layers of energetic thermoplastic elastomer (ETPE) based propellants and laminate the layers using heat and pressure. The faster burning layer is sandwiched by two slower burning layers to yield a three-layered slab configuration.
Another method is to feed two streams of thermoplastic-based propellants into an intricately designed die system through which multi-layered slab propellants are extruded. The co-extrusion die must be designed with many features that enable it to control heat, dimensions of individual extrudate layers, and pressure drop between inlet and outlet of the die. The extruded propellants can be post-processed further to be rolled into a scroll configuration or trimmed to pre-designed shapes based on the shape and volume of the targeted propulsion system. The two individual streams can be forced into the co-extrusion die using twin screw extruders, more traditional ram extruders, or a combination of both types.
Durand et al, describes in U.S. Patent Publication No. 20150284301, methods to prepare multi-layered propellant grains by simultaneously extruding one higher viscosity propellant formulation in the shape of a hollow cylinder and a second propellant having low viscosity that is injected into the interior of the first propellant formulation layer. The resulting propellant grain thus having different burn rates. Durand utilizes traditional nitrocellulose formulations that can be co-extruded to increase ballistic efficiency if the burn rate differential is large enough for a given gun system. These nitrocellulose formulations, however, have a tendency for the ingredients (e.g. plasticizer) in the formulation to migrate across the outer propellant layer and inner propellant layer until an equilibrium is reached, thus losing the ballistic benefits of a burn rate differential. This migration effect is also exacerbated at elevated temperatures or after aging.
The present disclosure addresses the migration of the propellant ingredients within the co-extruded propellant matrix.